**Question:** The influence of maternal smoking and LBW incidence is studied. Detailed smoking history is taken at first antenatal visit and smoking history and bih weight were studied later. The type of study is -

A. Cross-sectional study
B. Cohort study
C. Case-control study
D. Experimental study

**Correct Answer:** B. Cohort study

**Core Concept:** A cohort study is a type of observational study that follows a group of individuals over time to investigate the association between a risk factor (in this case, maternal smoking) and an outcome (low birth weight). This type of study focuses on the development or progression of a condition in a group of people who share common characteristics.

**Why the Correct Answer is Right:**

In this case, the correct answer is a **Cohort study** because the researchers are following a group of pregnant women with detailed smoking history taken at the first antenatal visit and then studying the association between smoking and low birth weight (LBW) later. The study aims to observe and analyze the relationship between a specific risk factor (smoking) and an outcome (LBW).

**Why Each Wrong Option is Incorrect:**

A. **Cross-sectional study:** This study design involves collecting data at a single point in time, which is not suitable for studying the progression of a condition over time, as in the case of investigating the influence of maternal smoking on LBW.

C. **Case-control study:** In a case-control study, the researchers compare smokers and non-smokers after identifying cases (pregnant women with LBW babies) and controls (pregnant women with normal birth weight babies). This type of study is not suitable for studying the effect of risk factors on outcomes over time like in the given question.

D. **Experimental study:** An experimental study involves manipulating a variable (e.g., smoking cessation) and observing the effects on an outcome (LBW). This type of study is not suitable for studying the influence of a risk factor on the outcome without intervening (just observing) as in the given question.

**Why the Correct Answer is Right:**

A cohort study is right for this question because it allows researchers to follow a group of pregnant women over time to observe the association between smoking and LBW. The study will track the smoking behavior of participants and analyze the occurrence of LBW in relation to smoking patterns. This method allows for a causal relationship to be established, as it does not involve intervening on the risk factor but only observing the outcome.

**Why Each Wrong Option is Incorrect:**

1. A cross-sectional study involves collecting data at a single point in time, making it unsuitable for studying the progression of a risk factor on an outcome over time.
2. A case-control study compares smokers and non-smokers after identifying cases (pregnant women with LBW babies) and controls (pregnant women with normal birth weight babies). This approach does not allow for observing the progression of smoking on LBW, as it involves comparing groups after the outcome has occurred, rather than observing the progression of the risk factor on the outcome